1. Field of the Invention
The invention relates generally to the field of drilling wellbores through subsurface rock formations. More specifically, the invention relates to methods for determining and maintaining optimum wellbore fluid pressure during drilling and using wellbore fluid pressure response measurements to determine formation integrity and optimal drilling operating parameters.
2. Background Art
The exploration for and production of hydrocarbons from subsurface rock formations requires devices to reach and extract the hydrocarbons from the rock formations. Such devices are typically wellbores drilled from the Earth's surface to the hydrocarbon-bearing rock formations in the subsurface. The wellbores are drilled using a drilling rig. In its simplest form, a drilling rig is a device used to support a drill bit mounted on the end of a pipe known as a “drill string.” A drill string is typically formed from lengths of drill pipe or similar tubular segments threadedly connected end to end. The drill string is longitudinally supported by the drilling rig structure at the surface, and may be rotated by devices associated with the drilling rig such as a top drive, or kelly/kelly busing assembly. A drilling fluid made up of a base fluid, typically water or oil, and various additives is pumped down a central opening in the drill string. The fluid exits the drill string through openings called “jets” in the body of the rotating drill bit. The drilling fluid then circulates back toward the surface in an annular space formed between the wellbore wall and the drill string, carrying the cuttings from the drill bit so as to clean the wellbore. The drilling fluid is also formulated such that the fluid pressure applied by the drilling fluid is typically greater than surrounding formation fluid pressure, thereby preventing formation fluids from entering the wellbore and collapse of the wellbore. However, such formulation also must provide that the hydrostatic pressure does not exceed the pressure at which the formations exposed by the wellbore will fail (fracture).
It is known in the art that the actual pressure exerted by the drilling fluid (“hydrodynamic pressure”) is related to its formulation as explained above, its other rheological properties, such as viscosity, and the rate at which the drilling fluid is moved through the drill string into the wellbore. It is also known in the art that by suitable control over the discharge of drilling fluid from the wellbore through the annular space, it is possible to exert pressure in the annular space between the drill string and the wellbore wall that exceeds the hydrostatic and hydrodynamic pressures by a selected amount. There have been developed a number of drilling systems called “dynamic annular pressure control” (DAPC) systems that perform the foregoing fluid discharge control. One such system is disclosed, for example, in U.S. Pat. No. 6,904,981 issued to van Riet and assigned to the assignee of the present invention. The DAPC system disclosed in the '981 patent includes a fluid backpressure system in which fluid discharge from the borehole is selectively controlled to maintain a selected pressure at the bottom of the borehole, and fluid is pumped down the drilling fluid return system to maintain annulus pressure during times when the mud pumps are turned off (and no mud is pumped through the drill string). A pressure monitoring system is further provided to monitor detected borehole pressures, model expected borehole pressures for further drilling and to control the fluid backpressure system. U.S. Pat. No. 7,395,878 issued to Reitsma et al. and assigned to the assignee of the present invention describes a different form of DAPC system.
The formulation of the drilling fluid and when used, supplemental control over the fluid discharge such as by using a DAPC system, are intended to provide a selected fluid pressure in the wellbore during drilling. Such fluid pressure is, as explained above, selected so that fluid pressure from the pore spaces of certain subsurface formations does not enter the wellbore, so that the wellbore remains mechanically stable during continued drilling operations, and so that exposed rock formation are not hydraulically fractured during drilling operations. DAPC systems, in particular, provide increased ability to control the fluid pressure in the wellbore during drilling operations without the need to reformulate the drilling fluid extensively. As explained in the patents referenced above, using DAPC systems may also enable drilling wellbores through formations having fluid pressures and fracture pressures such that drilling using only formulated drilling fluid and uncontrolled fluid discharge from the wellbore is essentially impossible.
Selection of the correct wellbore fluid pressure, even when using DAPC systems, however, requires at least prior estimation of the fluid pressure and the fracture pressures of the formations being drilling. Techniques known in the art for estimating such pressures include analysis of seismic surveys and gravity surveys. Other techniques may include refining estimates made from seismic and gravity surveys using actual drilling measurements and/or fluid pressure measurements from nearby wellbores. Irrespective of the techniques used to estimate formation fluid pressures and fracture pressures, the actual fluid pressures and fracture pressures encountered during drilling the wellbore may be different from those predicted or estimated. Inaccurate estimation of the fluid pressures and the fracture pressures may result in reduced drilling efficiency, increased risk of formation fracturing, increased risk of wellbore collapse, increased risk of drilling faults such as the pipe string becoming stuck in the wellbore, and increased risk of setting protective pipe or casing at incorrect depths with regard to the actual formation fluid pressures and fracture pressures.
There is a need for techniques to estimate the formation pore fluid pressures and formation fracture pressures while drilling, in order to better define formation integrity for correct casing depth selection and in order to better select drilling operating parameters for efficient drilling.